Vision Measuring Systems Advance Noncontact Dimensional Measuring Technology
Today’s vision measurement systems and microscopes use various technologies for noncontact measurement. This article discusses five principal technologies that may be used to “find the edge.”
In all aspects of life, it is said that change happens on the edge. Vision measuring systems are essentially designed to “find the edge” and define it relative to any process that might be affecting or changing it.
In the past, noncontact measuring systems could be quite subjective. Multiple technicians would rarely achieve the same results using a single technology. Current-generation, noncontact system platforms are not only better at finding the edge, but also at delivering a competitive edge to manufacturers and materials scientists. These are the people who design, test and measure components engineered to micron-scale tolerances. To put this scale into perspective, it’s useful to note the diameter of an average red blood cell is about 7.2 microns, and a human hair may range in diameter from 40 to 120 microns. At the next level of measurement, a nanometer is 1/1,000 of a micron.
The push for ever smaller, ever faster technology is driving advances in vision measuring systems that can deliver increasingly fine results more quickly.
Resolution capabilities continue to improve, with results such as higher resolutions at longer working distances, improved transmittal characteristics of light waves, the reduction of illumination “noise,” and higher numerical aperture values.
This allows for the resolving of micron-level features that were once lost in the poor quality of images.
Today’s noncontact dimensional measurement technology takes its users to the edge of discovery through an ever-increasing sophistication of sensor technology. In this article, we will review some features and benefits of this technology.
Noncontact dimensional measurement systems provide data that is not otherwise attainable in a reasonable amount of time. With almost all companies running lean, automating this type of measurement can improve quality while saving time and money.
As a category of automated devices, noncontact dimensional measurement systems include vision-sensing technology in one form or another of charge-coupled device (CCD), light, lenses and analytical software. A CCD is a light-sensitive integrated circuit that stores and displays the data for an image. A high-quality CCD can produce an image in extremely dim light, as its resolution does not deteriorate when the illumination intensity is low.
Today’s vision measurement systems and microscopes use various principles of technology. Although there are many methods and modes of achieving noncontact measurement, this article discusses five key technologies that may be used individually or in combination to “find the edge.” Many of these technology principles are not new, but with the advances in automation, software and computer systems, these tools have become very powerful and widely used across many industries.
Manufacturing categories benefitting from the various noncontact measurement technologies include semiconductors, automotive, aerospace, telecommunications, LEDs and MEMS. Energy industries such as solar and wind power generation also use these technologies to achieve a range of benefits.
To determine the best technology for your particular application, consult with a product specialist or an applications engineer.
Originally developed for applications in the life sciences, confocal technologies are now commonly used in a variety of industrial metrology applications. The confocal method uses point illumination and a spatial pinhole to eliminate the out-of-focus signal coming back from the specimen.
One popular method of confocal technology uses a Chromatic Point Sensor (CPS) to measure light displacement. By using an objective lens through which different colors of light focus at different points, the peak of the wavelengths can be detected by a spectrometer and converted to a height calculation.
The confocal method is ideal for measuring thickness of translucent substrates, mirrored and diffused surfaces. In general, confocal systems can produce higher-quality images than wide-field fluorescing microscopes because of the ability to remove out-of-focus results. Its primary advantage is having a high lateral resolution, while its primary disadvantage is the relatively small scan area. Higher lateral resolution leads to higher resolution of the 3-D reconstructed results. The small scan area can have an effect on the cycle times required to render an entire surface.
Typical industry segments using confocal scanning technology include the manufacture of medical devices (for example, micro-lens arrays) and electronics (for example, hard-drive suspension heads and integrated circuit boards).
White light interferometry (WLI)
WLI is a noncontact optical method for surface height measurement on 3-D structures of various profiles, providing resolutions down to the single-digit nanometer range. This is accomplished by splitting a beam of white light. Part of the beam will contact a reference mirror, while the other will contact the work piece. These two beams will interfere—hence the name, “interferometry”—with each other, creating light and dark bands. When a portion of the specimen is in focus, the bands are recorded onto a CCD camera, while the software reconstructs the 3-D data set. The magnification also can be increased to measure the overall surface profile at surface roughness resolutions.
While WLI achieves very high Z-axis resolution (1-10 nm), it tends to be a slower technology than confocal. It is, however, faster than other methods such as contact profile systems that can damage the surface, or atomic-force microscopy that can achieve higher resolution but at slower speeds.
WLI applications include integrated circuitry design, solar cell surfaces, photo spacers, fiber optics, semiconductors and microelectromechanical systems (MEMS) design and measurement. MEMS are very small devices containing components between 1 and 100 microns in size.
A primary advantage of WLI is that it provides very high vertical resolution, as well as increased throughput by its ability to perform area scanning vs. single-point scanning.
A disadvantage is that WLI is highly dependent on the sample’s refractive and optical properties.
A proprietary technology for high-speed 3-D reconstruction of pixel-based images, Points from Focus (PFF) provides resolution down to 50 nanometers. Relatively speaking, this is a low-resolution method for measuring the Z axis, but the speed of PFF makes it an excellent technology choice for measuring components such as diamond chips, fastener tips, surgical screws and a variety of other manufactured micro-components.
The primary advantages of PFF are its relatively low cost, its speed and its ability to integrate with existing optics on a vision system. Some of the disadvantages of PFF are lower resolution than other technologies and its inability to measure transparent materials or mirrored surfaces. It is highly dependent on optical properties of the specimen.
A more recently developed technology, structured illumination will continue being refined for far greater capabilities and super-resolution levels than are available today. With the increased speeds of image capture devices, image processors (such as frame grabber boards) and computer systems, processing massive amounts of raw images will not be the bottleneck. Structured illumination microscopy provides resolution capability similar to WLI technology. This technology is currently used on biological specimens because of its optical sectioning capability, which measures focal planes deep into a specimen that would otherwise require manual sectioning or cutting.
Among its advantages, structured illumination is able to create contrast on surfaces that have no natural contrast. It also can measure a greater depth of field and a larger field of view than other technologies. Its ability to measure a large field of view, however, is tempered by the number of raw images that are captured. This results in higher resolution but slower measuring. Its use is limited by the optical properties of the specimen.
Scanning lasers (single point)
Scanning laser technology has improved and evolved since its first applications, and now includes a variety of methods. One of those methods is known as point scanning. Analogous to a record player needle, a point scanning laser examines a surface for bumps and vibrations. A beam scanning at regular intervals in two dimensions can detect fine, structural details, resulting in high throughput. This single-point scanning method takes more time than confocal or WLI technologies.
Among the many applications of this technology is scanning manufactured components for the purpose of reverse engineering them. In this application, the point-scanning laser scans the entire surface and shape of the object at a sufficient resolution to build a CAD model.
Technology choice not always obvious
Consider a typical example of the application of vision measurement technology: wear-pattern analysis.
Wear patterns can be observed on certain types of machine tooling. The tooling can be measured before it is used in production and correlated to the parts that are being manufactured. After a certain amount of time or cycles, the tooling can be checked again for surface degradation or anomalies.
With the ability to trace the tooling back to the work piece, the user can determine with some certainty when the parts being manufacturing will start to fail. This enables precise forecasting of when tooling will need to be replaced, based on calculating the number of cycles that can be achieved before producing defective parts.
Of the scanning technologies available, either white light interferometry (WLI) or the confocal method of scanning the tooling surface will likely provide the best results for wear-pattern analysis. The ultimate choice of technology depends on the geometry of the surface and surface roughness. Some confocal systems have the ability to measure diffused surfaces at very steep angles (±80°) and mirrored surfaces up to angles of ±25°. WLI, on the other hand, can measure features with a higher aspect ratio (e.g., 10 microns deep and 10 microns wide).
Wear analysis can also be applied to the medical industry, particularly joint replacement manufacturers (hips, knees). Medical manufacturers of replacement components will reconstruct a surface geometry using a noncontact technology. Then the manufacturer will put the components through simulated walking and stresses of normal use over many cycles. They will then measure aftereffects, comparing the two surfaces based on the many data points obtained. Their purpose is to try to reduce wear and increase the lifecycle of the replacement component. Depending on the surface geometry and surface finish, either confocal or WLI technology can work for this application. For this very reason, some systems combine vision, white light interferometry and confocal measurement methods into one system.
Both WLI and confocal methods can pattern a 3-D surface, a crucial capability for performing wear-pattern analysis and a related application—defect analysis, which is particularly useful in the semiconductor industry.
Trends advancing vision measuring systems
From machining to electronics, we have seen a push for ever smaller, ever faster technology, and this trend is driving advances in the development and application of vision measuring systems that can deliver increasingly fine results more quickly.
Spurred by advances in lens technology, resolution capabilities continue to improve, with results such as higher resolutions at longer working distances, improved transmittal characteristics of light waves (minimizing light loss), the reduction of illumination “noise,” and higher numerical aperture values. This allows for the resolving of micron-level features that were once lost in the poor quality of images. Many times an operator would increase light or signal from a sensor to resolve the image, but would inherently increase the noise as well.
Speed is increased partly through automation, which also allows operators with less technical training to deliver precise, repeatable levels of accuracy. Today, the metrologists and technicians who use noncontact dimensional measuring systems possess widely varied levels of education, experience and skill. In our experience, the enterprises that get the best performance and overall value from their vision systems are those that have an internal “champion of the system”—an individual with metrology skills—who can record a measurement routine.
The measurement routine will then allow for easy playback of the automation to facilitate optimal use by all of the operators, regardless of their backgrounds.
The integration of advanced, easy-to-use analytical software provides improved data handling, accurate automation of the vision sensors, auto-adjusting and auto-correction of the images, and the ability to capture and manipulate ever-increasing numbers of data points. The capture and analysis of 3-D point data and images requires the use of powerful analytical and computational software to handle the higher volumes of data (thousands of data points within seconds) needed to accurately measure characteristics such as slope, topography, warpage and surface details. Important benefits of using programmable software include increased accuracy, the elimination of operator subjectivity, immediate calculation of summary data and statistics, creation of custom reports and the ability to save configurations for repeated, systematic use. The advancement of automated lighting techniques reduces operator subjectivity in determining optimum lighting settings.
A corollary trend to this is the enhancement of older sensing and imaging technologies by integrating them with microprocessors and computational software to capture and process data at high speeds.
Understand your choices, find your competitive edge
We are living in what may be a golden age for optical measurement systems and technologies. Lens optics continue to advance. Software handles larger data sets in shorter time. Automated systems can correct for part variations on the fly. Camera technology is getting smarter.
Understand your choices of noncontact dimensional measurement technologies and how each relates to your particular requirements. It’s all about “finding the edge”—and achieving a competitive edge.